Closed-loop systems are employed in a wide variety of applications to maintain an output at some desired value. The output is produced by a forward section of the system in response to a system input. To prevent changes in the system input from causing undesired deviations in the system output, a feedback section is employed. More particularly, the feedback section monitors the system output and produces a feedback signal that is combined with the system input to adjust the operation of the forward circuit and restore the system output to the desired condition.
Once the output signal of the closed-loop system is at the desired condition, the effect that minor deviations of the input signal have on the output signal can be easily sensed and the input signal corrected accordingly. In this situation, the system is able to maintain the desired output signal and in applications including polarization combination and phase locking the system is said to be in a state of "lock." If the input signal changes more drastically, however, the feedback section may be unable to make the corrections necessary to immediately restore the output signal to the desired level. In this condition, the system is said to be "out-of-lock" and the process of restoring the system to lock is known as "pull-in." When the input signal temporarily disappears, or when the system is first activated, the difference between the output signal produced and the desired output signal may be so great that the system is unable to pull-in, or adjust the input signal adequately to restore proper operation.
In such situations, it is necessary to sense the out-of-lock condition and provide some mechanism for bringing the system back into lock. This can be accomplished, for example, by causing the feedback signal to sweep through a range of values until the input signal is adjusted to a level at which the desired output signal is produced, and lock achieved. Then, with the system locked, this sweep or search function is disabled and standard closed-loop operation of the system resumes.
The enhancement of the system's pull-in capability, however, should not detract from other system characteristics. For example, unduly complex circuitry should not be required to perform the lock sensing or search operations. In addition, system performance should not be degraded via decreased loop sensitivity or noise immunity.
One prior art implementation of a search oscillator used to enhance the pull-in characteristics of a closed-loop system is shown in FIG. 1. The oscillator includes a first operational amplifier U.sub.1 having its inverting input coupled to an input signal V.sub.in by a resistor R.sub.1, and its noninverting input coupled to ground. The inverting input is also connected to a negative supply by a resistor R.sub.2. The output terminal of operational amplifier U.sub.1 provides an output signal V.sub.out. A capacitor C.sub.1 is connected between the inverting input and the output of operational amplifier U.sub.1.
The oscillator also includes a second operational amplifier U.sub.2, diode D.sub.1, and resistors R.sub.3 and R.sub.4 connected between the inverting input and the output of amplifier U.sub.1 in the following manner. The noninverting input of amplifier U.sub.2 is connected to the output of amplifier U.sub.1 by resistor R.sub.3. The inverting input of amplifier U.sub.2 is connected to ground. The output of amplifier U.sub.2 is connected to the positive side of diode D.sub.1, while the negative side of diode D.sub.1 is coupled to the inverting input of amplifier U.sub.1. Resistor R.sub.4 is connected between the noninverting input and output of amplifier U.sub.2.
Discussing now the operation of the search oscillator shown in FIG. 1, as will be appreciated, amplifier U.sub.1 is employed as an integrator and amplifier U.sub.2 is employed as a comparator. Together, they sense the out-of-lock condition of the closed-loop system and initiate sweeping of, for example, a system feedback signal to accomplish pull-in. A key feature of the oscillator is its use of the negative supply and resistor R.sub.2 to produce an offset current I.sub.offset. To illustrate the manner in which this offset current allows the oscillator to sense the out-of-lock condition and initiate sweeping, an example is provided.
Consider the situation in which the input voltage V.sub.in is absent. As will be appreciated, the current flowing through capacitor C.sub.1 is equal to the offset current I.sub.offset in this situation. Because the current flowing through a capacitor is proportional to the rate of the change of the voltage across the capacitor, the offset current I.sub.offset thus determines both the rate and direction of the change in the output V.sub.out of amplifier U.sub.1. As a result, in the arrangement shown, V.sub.out will rise slowly, sweeping across all required lock conditions.
The output V.sub.out of amplifier U.sub.1 is, in turn, applied to the noninverting input of amplifier U.sub.2. As will be appreciated, before V.sub.out began to rise, the low level signal applied to the noninverting input of amplifier U.sub.2 would have maintained U.sub.2 OFF and its output V.sub.1 low. As V.sub.out rises, however, the voltage applied to the noninverting input of U.sub.2 will also rise. This voltage, which is set by the voltage divider formed by resistors R.sub.3 and R.sub.4 between the low voltage V.sub.1 and the rising voltage V.sub.out, will at some point exceed the ground level applied to the inverting input of amplifier U.sub.2.
At this point, V.sub.out is at a first threshold level V.sub.a and amplifier U.sub.2 will switch ON, forward-biasing diode D.sub.1. As the charge on capacitor C.sub.2 increases, the current output by amplifier U.sub.2 will become adequate to overcome the offset current I.sub.offset. As a result, the output of amplifier U.sub.1 will drop low, along a line whose slope and direction are a function of the current output by amplifier U.sub.2 in excess of the offset current I.sub.offset.
As V.sub.out falls, the voltage divider formed by resistors R.sub.s and R.sub.f between the falling output V.sub.out and the high output V.sub.1 of amplifier U.sub.2 will eventually cause the voltage applied to the noninverting input of amplifier U.sub.2 to fall back below the ground level applied to the inverting input. At this point V.sub.out is at a second threshold V.sub.b. The cycle is then ready to repeat.
In this manner, the oscillator effectively detects the out-of-lock condition and causes the output V.sub.out of amplifier U.sub.1 to be swept through a signal range between the thresholds V.sub.a and V.sub.b of comparator U.sub.2. This range is roughly centered about ground.
The sweep function of the search oscillator shown in FIG. 1 is disabled in the following manner. For example, assume the circuit is locked on an incoming signal V.sub.in, having a magnitude that is sufficient to overcome the offset current I.sub.offset. In this situation, amplifier U.sub.1 functions as an integrator and compensates the feedback loop for stability. Because the output V.sub.out of amplifier U.sub.1 is no longer sweeping in response to the constant current I.sub.offset, comparator U.sub.2 is kept OFF and diode D.sub.1 back-biased. As a result, the sweep function is effectively disabled.
The search oscillator shown in FIG. 1 has the advantage of requiring only one limit-sensing amplifier U.sub.2 because the offset current I.sub.offset is greater than any inherent offset for amplifier U.sub.1, ensuring that the sweep will always occur with one polarity, e.g., negative-to-positive. As a result, only one sensor, having a positive trigger threshold, is required. This search oscillator is, however, able to lock onto only signals large enough to overcome the offset current I.sub.offset and hence disable the sweep function. In other words, the offset current I.sub.offset degrades the sensitivity of the feedback loop. In addition, because the loop must overcome the offset current I.sub.offset, noise is more likely to cause the oscillator to fall out-of-lock.
In addition to the search oscillator shown in FIG. 1, other prior art arrangements have been developed. For example, some search schemes involve the use of detectors that sense the magnitude of the loop error. These schemes also degrade loop sensitivity due to the detector thresholds. In addition, more complicated lock-sensing schemes have been devised, including circuits employing phase lock loops with extra phase detectors.
In view of the preceding remarks, it would be desirable to produce a search oscillator that is capable of sensing when the system is out-of-lock and enhancing the system's pull-in capability, without undue complexity and without degrading loop sensitivity or the noise immunity of the system.